I'm pretty traditional in this case, and I think a child should have it's father's surname. If he's a decent guy, I think he deserves to have his child take his surname. If he's a deadbeat and actually bad for this child, then I would say it's your call. If he hasn't actually done anything against this new baby, or against your daughter, then I wouldn't take this away from him.
Also, if you give this new child your surname, it may imply that it's father is a deatbeat good for nothing, even if he's not. It may also allow people to think they have different fathers, which they don't, and yes, your children would eventually ask why they weren't given the same surname. I just think it's adding a lot of extra confusion if you don't give this baby his name
And as a secondary note (not that many people care about this part), but I trace my family history in my spare time, and to save confusion for anyone in the future, I think it's important to name a child after it's father (surname that is; not first name)